The invention lies in the field of semiconductor technology. Specifically, the invention relates to a thyristor comprising a semiconductor body with
an anode-side base zone of the first conductivity type and at least one cathode-side base zone of the opposite, second conductivity type;
anode-side and cathode-side emitter zones; and
at least one region in the cathode-side base zone whose geometry gives it a reduced breakdown voltage as compared with the remaining regions in the cathode-side base zone and the edge of the semiconductor body.
In high-voltage systems, a number of thyristors are generally connected in series. They need to be continually fired simultaneously. If one of the thyristors fires later, then it will have virtually the whole voltage across it, and the thyristor will be destroyed. Efforts are therefore being made to develop thyristors which can be triggered xe2x80x9cat breakover,xe2x80x9d i.e. without base current. Such thyristors generally have a central region which has a lower breakdown voltage than the remaining region and the edge. If the voltage across the thyristor rises, this region enters avalanche breakdown and the breakdown current can trigger the thyristor directly or via one or more auxiliary thyristor structures.
The breakdown region may, for example, be produced by virtue of the fact that the cathode-side base zone has a cutout inside which a thinner layer of the same conductivity type is arranged on the surface of the semiconductor body. The pn junction between the anode-side base zone and the cathode-side base zone then has a defined radius of curvature at the transition from the horizontal to the cutout, this radius of curvature experiencing a field strength that is higher than a planar pn junction. Hence, it is preferable for the thyristor to break down at the curve. A structure of this generic type has been described, for example, in the article xe2x80x9cDesign consideration for high-power, overvoltage self-protected thyristorxe2x80x9d by Ohashi, Yoshida, Yamaguchi, Akagi, published in IPEC-Tokyo 1983, pages 550-58, particularly with reference to FIG. 1b. 
The breakdown characteristics of the region depend on the shape of the pn junction of the cathode-side base zone. U.S. Pat. No. 5,455,434 to Pfirsch (cf. DE 42 15 378 C1≅EP 0 572 826 A1) discloses a further thyristor of this generic type which has regions of reduced breakdown voltage. These regions of reduced breakdown voltage are very effective and can be reproduced easily.
The breakover voltage of thyristors having integrated overvoltage protection is highly temperature-dependent there, however. Reasons for this are, firstly, the breakdown voltage, which rises with temperature, and the emitter/collector gain xcex1pnp, which increases with temperature. At high temperatures, the transistor gain xcex1pnp amplifies the reverse current in such a manner that the thyristor is triggered prematurely at a lower breakover voltage than intended. This can cause the thyristor to be triggered prematurely unintentionally.
European published application EP 423 721 describes a thyristor having a deformed region with a recombination zone. The deformed region has a reduced breakdown voltage on account of its geometry. Further semiconductor components of this generic type which are designed as thyristors and have a reduced breakdown voltage are described in international PCT publication WO 92/17907 and in European application EP 0 572 826.
It is accordingly an object of the invention to provide a thyristor with a breakdown region, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and in which the breakover voltage of the thyristor is largely independent of temperature in the temperature range of the thyristor operation.
With the foregoing and other objects in view there is provided, in accordance with the invention, a thyristor, comprising:
a semiconductor body formed with
an anode-side base zone of a first conductivity type and at least one cathode-side base zone of a second conductivity type opposite the first conductivity type;
anode-side and cathode-side emitter zones;
a region in the cathode-side base zone having a geometry defining a reduced breakdown voltage as compared with remaining regions in the cathode-side base zone and an edge of the semiconductor body;
a recombination zone formed at the anode below the region of reduced breakdown voltage, the recombination zone having free charge carriers with a reduced lifetime; and
the regions in the cathode-side base zone having a defined geometry with the following features:
a central region of the cathode-side base zone having a cutout formed therein and including a thin layer of the second conductivity type inside the cutout on a surface of the semiconductor body, the layer being relatively thinner than and connected to the cathode-side base zone;
an additional zone of the second conductivity type formed in the cutout adjacent the thin layer; and
the additional zone, as viewed from the cathode-side base zone, being at least partly concave.
In other words, the recombination centers which reduce the lifetime of the free charge carriers are provided here at the anode beneath the zone of reduced breakdown voltage.
In accordance with an added feature of the invention, the recombination zone is formed with crystal lattice defects produced by irradiation. In a preferred embodiment, the crystal lattice defects are Frenkel defects or Schottky defects.
In accordance with an additional feature of the invention, the regions in the cathode-side base zone, the cathode-side base zone, and the cathode-side emitter zones are circular in a plane of the surface of the semiconductor body.
In accordance with a further feature of the invention, the thyristor is an annular thyristor.
In accordance with another feature of the invention, a doping concentration of the thin layer is very much higher than a doping concentration of the cathode-side base zone and of the additional layer.
In other words, the recombination zone is essentially composed of defects which are produced by irradiation with nondoping, high-energy particles. The defects are usually Frenkel defects or Schottky defects, which are produced during irradiation of the semiconductor body with xcex1-particles or protons. Other defects are also conceivable, however. A relatively low dose of 1010 to 1012 cmxe2x88x922 is used for the irradiation, since the crystal should not be damaged too severely by the irradiation.
In accordance with again an added feature of the invention, the anode-side base zone has, in a vertical direction defined by a main surface normal to the semiconductor body, an inhomogeneous density distribution at recombination and generation centers for free charge carriers. In a preferred embodiment, the density of the recombination and generation centers within a first region of the anode-side base zone is higher than in regions of the anode-side base zone adjoining one another in the vertical direction on both sides and in each case extend as far as an adjacent pn junction.
In accordance with again an additional feature of the invention, the anode-side base zone has a vertical thickness dB, and a dimension b of the first region satisfies the condition dB less than b less than 2dB in a lateral direction.
In accordance with again another feature of the invention, a vertical position of the first region within the anode-side base zone is chosen such that a space-charge zone of the pn junction associated with the two base zones extends as far as the first region when there is a predetermined potential difference between a cathode potential and an anode potential.
Preferably, the predetermined potential difference is approximately equivalent to a reduced breakdown voltage.
The further zone protects the surface of the thyristor against surface charges. This zone has a correspondingly higher doping than the base zone and the zone of reduced breakdown voltage.
With the above and other objects in view there is also provided, in accordance with the invention, a method of producing the above-outlined thyristor. In particular, the method is concerned with the fabrication of the recombination zone and comprises the following steps:
masking the anode of the semiconductor body, preferably with a perforated metal screen;
irradiating the anode, specifically with nondoping, high-energy particles such as protons or xcex1-particles; and
subsequently heat treating for stabilizing the recombination zone.
If the irradiation is with protons, then the dosage is adjusted to a range from 1011 to 1013 cmxe2x88x922. If the irradiating particles are xcex1-particles, then the dose is between 1010 and 1012 cmxe2x88x922.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a thyristor with breakdown region, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.